1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device.
2. Description of Related Art
Conventionally, in LSIs, a MOS transistor having a thick gate insulating film for high voltage and a MOS transistor having a thin gate insulating film for low voltage have been formed. In recent years, there has been an increasing trend toward using a lower supply voltage in the LSIs. Where the supply voltage is 1.8 V, for example, the difference is small between the supply voltage and a typically used threshold voltage of about 0.6 V. Therefore, in the case of differential amplifier circuits, for example, circuits that stably operate at low supply voltage cannot be formed without transistors having a low threshold voltage.
However, it is not possible to reduce the threshold voltage of all transistors, because the leakage current of the MOS transistors increases as the threshold voltage decreases. For this reason, it is preferable that only part of the MOS transistors for low voltage has a low threshold voltage.
For the fabrication of MOS transistors having two types of threshold voltages (i.e., high threshold voltage and low threshold voltage) in a single semiconductor substrate, there is known a method in which an ion-implantation step for adjusting the threshold voltage is added. However, this method has a problem that the number of steps for photolithography and ion-implantation increases in the fabrication process.
Japanese Unexamined Patent Publication No. HEI 06(1994)-283675 discloses a method in which a region for forming a transistor having a low threshold voltage is covered with a resist at the time of ion-implantation for well formation, so that a low impurity concentration region is formed under the covered region. In this method, wells are formed by oblique implantation, and adjustment of the threshold voltage of the transistor having a low threshold voltage is also performed by oblique implantation.
In this method, it is difficult to control the impurity concentration near the surface of a semiconductor substrate. Therefore, the threshold voltage of the transistor having a low threshold voltage tends to vary widely.